Engine mount systems using vacuum-driven switchable liquid-filled engine mounts are in practical use. The liquid-filled engine mounts are switchable to provide different vibration damping characteristics by selectively introducing negative pressure from the intake manifold of an engine or atmospheric pressure. By thus switching the pressure introduced therein, the engine mount can appropriately absorb the engine vibrations.
An engine mount system including such switchable liquid-filled engine mounts is disclosed in, for example, JP-A-2003-49892 wherein damping characteristics of the engine mounts are varied when the engine speed changes from an idle speed range to a high speed range or vice versa. Such disclosed engine mount system will be explained with reference to FIG. 12 hereof.
The engine mount system (designated at 300) shown in FIG. 12 includes vacuum-driven switchable liquid-filled engine mounts 302 (only one shown) attached to a vehicle body 303 for supporting an engine 301, and a solenoid valve (an electromagnetically switching valve) 304 connected to pressure introducing pipes of the engine mounts 302. The solenoid valve 304 communicates with the surrounding atmosphere and is connected to an intake manifold 305 of the engine 301 as a negative pressure source.
The solenoid valve 304 switches pressure to be introduced into the liquid-filled engine mounts 302 to vary the degree of firmness with which the engine mounts 302 support the engine 301. The degree of firmness will be referred to as “mount characteristic” or “vibration damping characteristic”. For example, when the engine 301 runs in an idle speed range, negative pressure is introduced into the engine mounts 302 to set the mount characteristics soft. When the engine 302 runs in a high speed range, atmospheric pressure is introduced into the engine mounts 302 to set the mount characteristics hard.
Plural (for example, four) vacuum-driven switchable liquid-filled engine mounts are used for the purpose of supporting an engine on a sub-frame (not shown). However, the engine mount system 300 is less suitable for this purpose because all the mount characteristics of the plural engine mounts 302 are changed or switched simultaneously. With this simultaneous switching, the mount characteristics of the engine mount system 300 vary abruptly. Due to this abrupt change, the mount characteristics can not correspond to the speed of the engine 301. As a result, efficient suppression of vibrations from the engine 301 can not be achieved. To address this problem, a technique is needed which prevents an abrupt change in the mount characteristics of the engine mounts when the pressure introduced in the engine mounts is switched, so as to efficiently absorb engine vibrations.
In the engine mount system 300, the one solenoid valve 304 switches the pressure in all the plural engine mounts 302 to the negative or atmospheric pressure. Thus, the vibration damping characteristics of the plural engine mounts 302 can assume only two states, that is, the vibration damping characteristics are all soft or all hard. As a result, inappropriate mount characteristics not corresponding to the speed of the engine would be inevitably used. Therefore, it is desirable to set a mount characteristic suitable for a speed range other than the idle speed range and the high speed range. Namely, an engine mount system is required which is capable of setting a mount characteristic of a vacuum-driven switchable liquid-filled engine mount in correspondence to a vehicular state.